Related Posts on AMP: Lower-priority filter hooks are removed

[micropat]

Filter hooks with a low-priority number of 41 or greater are removed on AMP pages when the Related Posts module is activated and setup.

Steps to reproduce the issue

1. Add the PHP code below to the site's functions.php or similar.
2. Activate & setup the official AMP plugin.
3. Observe that additional content is added to post content on an AMP page.
4. Activate & setup Jetpack's Related Posts module.
5. Observe that additional content is not added to post content on an AMP page.

add_filter( 'the_content', 'add_something_to_content_filter', 41 );

function add_something_to_content_filter( $content ) {
	$original_content   = $content;
	$add_before_content = 'SOMETHING BEFORE CONTENT.';
	$add_after_content  = 'SOMETHING AFTER CONTENT.';
	
	$content = $add_before_content . $original_content . $add_after_content;

	return $content;
}

What I expected

SOMETHING BEFORE CONTENT. should prepend content.
SOMETHING AFTER CONTENT. should append content.

What happened instead

No additional content is added to content.

[jeherve]

We've been looking at this as well, most likely thanks to a customer we have in common. :)

Internal reference: 4412-gh-jpop-issues

@kbrown9 has been spearheading this investigation so far, and here is what she's found so far:

This issue is caused by a bug in WordPress that was first reported ~10 years ago. Multiple tickets have been opened for this over the years, but a fix hasn't been accepted.

https://core.trac.wordpress.org/ticket/9968
https://core.trac.wordpress.org/ticket/40393
https://core.trac.wordpress.org/ticket/40685

Until this gets fixed in Core, I'll try to figure out a way to avoid this bug in the Related Posts feature.

[kbrown9]

Thanks for the bug report @micropat!

I just want to add an additional detail about this bug. If there are multiple filters with priorities greater than 40, only the first filter with a priority greater than 40 is skipped. All of the other filters are applied.

For example, if you try this code, you’ll see that 'add_something_to_content_filter2’ is applied.

add_filter( 'the_content', 'add_something_to_content_filter', 41 );

function add_something_to_content_filter( $content ) {
	$original_content   = $content;
	$add_before_content = 'SOMETHING BEFORE CONTENT.';
	$add_after_content  = 'SOMETHING AFTER CONTENT.';
	
	$content = $add_before_content . $original_content . $add_after_content;

	return $content;
}

add_filter( 'the_content', 'add_something_to_content_filter2', 42 );

function add_something_to_content_filter2( $content ) {
	$original_content   = $content;
	$add_before_content = 'SOMETHING BEFORE CONTENT 2 .';
	$add_after_content  = 'SOMETHING AFTER CONTENT 2 .';
	
	$content = $add_before_content . $original_content . $add_after_content;

	return $content;
}